Hygienic Physiology : with Special Reference to the Use of Alcoholic Drinks and Narcotics

by Joel Dorman Steele

[Illustration: The three classes of Levers, and also the foot as a
Lever.]

THE LEVERS OF THE BODY. [Footnote: A lever is a stiff bar resting on a point of support, called the fulcrum (F), and having connected with it a weight (W) to be lifted, and a power (P) to move it. There are three classes of levers according to the arrangement of the power, weight, and fulcrum. In the first class, the F is between the P and W; in the second, the W is between the P and F; and in the third, the P is between the W and F (Fig. 18). A pump handle is an example of the first; a lemon squeezer, of the second; and a pair of fire tongs, of the third. See "Popular Physics," pp. 81-83, for a full description of this subject, and for many illustrations.]—In producing the motions of the body, the muscles use the bones as levers. We see an illustration of the first class of levers in the movements of the head. The back or front of the head is the weight to be lifted, the backbone is the fulcrum on which the lever turns, and the muscles at the back or front of the neck exert the power by which we toss or bow the head.

FIG. 19.

[Illustration: The hand as a Lever of the third class.]

When we raise the body on tiptoe, we have an instance of the second class. Here, our toes resting on the ground form the fulcrum the muscles of the calf (gas-troc-ne'-mi-us, j and so-le'-us, Fig. 14), acting through the tendon of the heel, [Footnote: This is called the Tendon of Achilles (k, Fig. 14) and is so named because, as the fable runs, when Achilles was an infant his mother held him by the heel while she dipped him in the River Styx, whose water had the power of rendering one invulnerable to any weapon. His heel, not being wet, was his weak point, to which Paris directed the fatal arrow—"This tendon," says Mapother, "will bear one thousand pounds weight before it will break." The horse is said to be "hamstrung," and is rendered useless, when the Tendon of Achilles is cut. (see p. 284.)] are the power and the weight is borne by the ankle joint.

An illustration of the third class is found in lifting the hand from the elbow. The hand is the weight, the elbow the fulcrum, and the power is applied by the biceps muscle at its attachment to the radius (A, Fig. 19.) In this form of the lever there is great loss of force, because it is applied at such a distance from the weight, but there is a gain of velocity, since the hand moves so far by such a slight contraction of the muscle. The hand is required to perform quick motions, and therefore this mode of attachment is desirable.

The nearer the power is applied to the resistance, the more easily the work is done. In the lower jaw, for example, the jaw is the weight, the fulcrum is the hinge joint at the back, and the muscles (temporal, d, and the mas'-se'ter, e, Fig. 14) on each side are the power. [Footnote: We may feel the contraction of the masseter by placing our hand on the face when we work the jaw, while the temporal can be readily detected by putting the fingers on the temple while we are chewing. The tendon of the muscle (digastric)—one of those which open the jaw—passes through a pulley (c, Fig. 14) somewhat like the one in the eye.] They act much closer to the resistance than those in the hand, since here we desire force, and there, speed.

FIG. 20.

[Illustration: The Kneejoint; k, the patella; f, the tendon.]

THE ENLARGEMENT OF THE BONES AT THE JOINTS not only affords greater surface for the attachment of the muscles, as we have seen, but also enables them to work to better advantage. Thus, in Fig. 20 it is evident that a muscle acting in the line f b would not bend the lower limb so easily as if it were acting in the line f k, since in the former case its force would be about all spent in drawing the bones more closely together, while in the latter it would pull more nearly at a right angle. Thus the tendon f, by passing over the patella, which is itself pushed out by the protuberance b of the thigh bone, pulls at a larger angle, [Footnote: The chief use of the processes of the spine (Fig. 6) and other bones is, in the same way, to throw out the point on which the power acts as far from the fulcrum as possible. The projections of the ulna ("funny bone") behind the elbow, and that of the heel bone to which the Tendon of Achilles is attached, are excellent illustrations (Fig. 1).] and so the leg is thrown forward with ease in walking and with great force in kicking.